The present invention relates to high voltage audio amplifiers.
High voltage audio amplifiers are often used in fixed audio installations having large distances from the audio power amplifier to the loudspeaker. Such installations are often characterized by having several loudspeakers connected to the same amplifier output channel. To keep the loudspeaker transmission line power loss to reasonable low level, the voltage of the amplifier output is kept at a high level resulting in a low current and thus a low power loss in the long transmission line, since power equals the current squared multiplied with the transmission line resistance. To match the low impedance (4-8 Ohm) dynamic loudspeaker typically used in these installations, each single loudspeaker is equipped with a transformer that transforms the signal to a suitable low voltage level. Exactly the same principle is used when transporting energy from electrical power plants to the consumer 115/230 VAC power outlet. In this case the energy is transported hundreds of km through relatively thin high-voltage power lines passing through local transformer stations, where the voltage is transformed to the typical 115/230 VAC consumer levels.
The audio installation systems described above are often named Constant Voltage Amplifiers, high-voltage audio distribution systems or 70 V/100 V Amplifiers, and to reach the high output voltage requirement different techniques are used today. A very common approach is to use a traditional audio amplifier with limited voltage capabilities and equip that with a rather large audio transformer to convert the raw amplifier output to a higher voltage level. This has of course a price, size and weight penalty compared to a pure solid state solution. It should be mentioned that this transformer has to pass the full amount of power to several loudspeakers and the power capability of this output transformer should in principle equal the sum of the power capabilities of the transformers found in the loudspeaker chain connected. Typically, transformers have limited low frequency response, since the transformer easily runs into core saturation due to a high transformer current at low audio frequency.
Another technique to establish a high voltage amplifier output is to design a solid state amplifier that can produce the high voltage directly. With the rise of Class-D audio amplifier technologies, this technique is seen more and more. However, the voltage requirements to such designs are still quite tough with presently available power transistors. Aiming for 70 Vrms output capability, this requires transistors rated to higher than 100V for a full-bridge design and higher than 200V for a single-ended design. At these voltage levels, a discrete design is required since no monolithic power device exists that can sustain this voltage potential.
A few other techniques to overcome the issues with limited transistor voltage capabilities are known from prior art.
US Patent Application Publication No. 2008/0309406 A1 (Jonkman) describes a full-bridge design in which one of the two amplifier outputs is connected to ground. This requires that the amplifier bridge is powered by a floating power supply since both switching node outputs have to be able to take the position of both full positive rail and full negative rail, so if the amplifier bridge should operate as intended (seen from the load perspective), the supply voltage cannot relate to ground in this design. In the patent application it is mentioned that two such amplifiers can be bridged requiring two isolated power supplies.
U.S. Pat. No. 6,671,329 (Lenz) describes a high voltage gradient amplifier design for a magnetic resonance tomography apparatus. This design consists of a series connection of several PWM amplifiers each having individual floating power supplies.
Both of the designs mentioned above require an isolated power supply per full bridge along with electrically isolated control and feedbacks signal to and from each full bridge. Besides the mentioned control and feedback signals, several other signals often also need to be transmitted out from or into the isolated circuit, thus quickly requiring a very high number of isolated signal transfers. Such further signals that are often desired may include “status”, “warning” and “error” data between controller and power supplies, and between controller and amplifiers.